Yeah, it's a major win. That's the problem. Dissatisfied voters are the ones who go to the polls. There's a reason why the President almost always ends up losing seats in his first midterm, because the people who are dissatisfied that he won go out again, while those are stay home.
I'm not saying that Roe v. Wade should not have been overturned, and I think there are reasons to think that it might not hurt the GOP too much electorally (the problematic economy is a bigger issue, and the possibility of a violent overreaction from the libs). But I do think that it is a damper on GOP hopes. Of course, that would be true at any time, so this is probably the best time from a political standpoint to take the hit, several months before a midterm which looks really good for the GOP.